Design Realization lecture 14

John Canny/Dan Reznik10/9/03

Last Time Composites: Fiberglass, carbon fiber and

kevlar.

Hierarchical materials.

Cellular materials, honeycomb and foam.

This time Electronics

Voltage, Current, Ohm’s law Voltage is analogous to pressure, and is

measured naturally enough, in volts. Current is analogous to flow, and is measure

in amperes or amps for short. Direct current (DC) is a constant voltage, e.g.

a single C or D battery produces 1.5 volts. Alternating Current (AC) is a voltage that

reverse rapidly, at 60 cycles/second in the US. An electrical outlet gives 110 volts AC.

Voltage, Current, Ohm’s law Resistors are used to produce desired

voltage or current, independent of frequency. Resistance is measured in ohms, and the

current through a resistor satisfies Ohm’s law:

V = I R

I in ampsV in volts

Resistors Resistors have a power rating as well, ½, ¼, 1/8

watt etc. (P = V I) Resistors used to all look like this:

(axial lead type):

But increasingly are surface-mount:

Or grouped in chip packages:

Resistors Variable resistors are called

potentiometers:

Here’s a simple circuit, avoltage divider:

Note the ground and power supply symbols:

A potentiometer can actas a variable voltage divider, to control a voltage.

AC and frequency Alternating current most often has a

sinusoidal shape over time: The frequency is the

number of completecycles per second.

Its measured in Hertz (Hz).

Waveform is V = sin 2 f t

AC and Capacitors Capacitors are charge storage devices, but

don’t allow DC to flow. AC can flow because a little charge is stored

each cycle and returned. The current flow

increases with frequency.

Capacitor Construction Capacitors are sandwiches of dielectric

between two conductors. The dielectric is an insulator, usually a

polymer. Performance determined by “dielectric

constant” and electrical breakdown strength (kV/mm).

Capacitor Construction

Capacitor Reactance A capacitor limits AC current rather like a

resistor does. The reactance Z of the capacitor determines

how much current flows, V = Z I where:

C is the capacitance in Farads. A Farad is a huge unit. Most capacitors are

measured in micro-farads or pico-farads (10-12)

CfZ

21

Variable Capacitors Capacitors can be variable. Used for tuning: Radios, antennas, crystal oscillators (to drive

computers).

Inductors Inductors are coils of wire, sometimes around a

ferrite or iron core. The ferrite core is a composite with small

magnetic particles. Works at high frequencies where iron doesn’t.

Transformer Two coils of wire around the same magnetic

core create a transformer. An AC voltage in one coil induces a voltage in

the other. Ratio of voltages =

ratio of turns. (more turns = higher

voltage).

A simple R/C circuit This circuit is a voltage divider, with one leg

which is a capacitor, one a resistor. Discuss what “high-pass” and “low-pass”

would mean in this circuit.

Amplifiers Amplifiers are an important class of active

component (resistors, capacitors and inductors are passive – they cant strengthen a signal).

Amplifiers boost small signals from radio antennas, microphones, sensors etc. to larger values.

Ex: stereo amplifier. There is a popular component for building

amplifiers called an Operational Amplifier (Op-Amp).

Inverting Amplifier Here is a basic inverting amplifier. The gain (ratio of Vo to Vi) is - Rf / Ri The OpAmp has very high gain, which makes

it change output until its two inputs are nearly equal – you can assume they are.

Non-Inverting Amplifier Here is a basic non-inverting amplifier. The gain (ratio of Vo to Vi) is (Rf + Rg) / Rg